1. Field of the Invention
This application relates generally to dielectric materials including low dielectric constant polymer materials and more particularly to solvent systems for use in formulation and clean up processing of such materials.
2. Description of the Related Art
As the dimension of the interconnect design rules for integrated circuits (IC) undergoes progressive shrinkage to sub-quarter micron metal spacing, the use of polymer dielectrics that minimize capacitance and reduce power consumption and cross talk, while increasing signal propagation speed becomes a necessity. The dielectric materials must possess dielectric constants no higher than 3.0 and should have dielectric constants as low as possible toward a theoretical limit of 1.0. The practical expectation for polymer dielectrics is in the range of 2.2 to 3.0. Both inorganic and organic polymer dielectrics are potentially useful. For organic dielectrics, the glass transition temperature is an important consideration. The organic dielectrics must have glass transition temperatures above 300° C. and as high as possible toward 450° C., a value determined by the thermal stability of organic polymers. The organic dielectrics should also be easily processed, preferably, by standard spin-bake-cure processing techniques. The organic dielectrics should also be free from moisture and out-gassing problems, in addition to having expected adhesive and gap-filling qualities, and dimensional stability towards thermal cycling, etching, and chemical mechanical polishing processes.
Arylene ether polymers have been identified as organic dielectric materials. Arylene ether polymers include poly(arylene ethers) (PAE) such as the FLARE™ material of AlliedSignal Inc. and the VELOX™ material of Schumacher. Other useful arylene ether polymers include poly(arylene ether ether ketone) (PAEEK), poly(arylene ether ether acetylene) (PAEEA), poly(arylene ether ether acetylene ether ether ketone) (PAEEAEEK), poly(arylene ether ether acetylene ketone) (PAEEAK), and poly(naphthylene ether) (PNE) comprising different polymer designs that include homopolymers, block or random copolymers, polymer blends, interpenetrating polymer networks (IPNs), and semi-interpenetrating polymer networks (SIPN)s. Additional examples of organic dielectric materials in current use include the polymeric material obtained from the phenyl-ethynylated aromatic monomer provided by the Dow Chemical Company under the tradename SiLK™.
Organosilicon polymers have also been identified as low dielectric constant materials. In particular, siloxane based resins including hydridosiloxane resins, organohydridosiloxane resins, and spin-on-glass siloxanes and silsesquioxanes are used as dielectric layers. Other classes of organosilicon materials include poly(perhydrido)silazanes and nanoporous dielectric silica coatings formed from liquid alkoxysilane compositions.
Taking advantage of the low dielectric property of organic-containing polymeric materials requires the IC industry to continue to shift its processing paradigm. Processing approaches, such as the use of spin-coating, require selection of appropriate solvents for formulation of the coating solution, and for cleaning, edge-bead removal, and wafer backside rinsing. Desirable formulations will provide spin-coated polymer dielectric films with excellent uniformity, a wide thickness range from hundreds of angstroms to hundreds of microns, very low out-gassing at high temperature, excellent gap-filling to 0.1 micron, excellent local, regional and global planarization, and ease of wafer edge bead removal and wafer backside rinsing. In addition, the dielectric polymer solution should be easily filtered to minimize its manufacturing cost.
While conventional alcoholic solvents, familiar to IC engineers, are obvious solvent candidates, they cannot necessarily be applied to organic materials. Ketonyl and other aprotic solvents have been used for photoresists and polymer dielectrics. In particular, cyclic ketonyl solvents are commonly used as solvents for arylene ether dielectrics. However, cyclic ketones normally are not as miscible with most arylene ether polymer dielectrics as would be desired and the spin-on solutions formulated from these solvents usually yield some extent of striation on the spin-coated film, especially for films with thicknesses greater than 1.5 micron. Serious striation could cause inadequate gap-filling, problems in adhesion of the dielectric film with a substrate and other problems. Additionally, cyclic ketonyl solvents have varying degrees of moisture, pH, and photosensitivities, often exacerbated by heat. For example, cyclopentanone is significantly more sensitive than first thought toward low pH, in addition to its well-known sensitivity toward light, moisture, and high pH. Cyclohexanone is more stable than cyclopentanone and has been a fair solvent for photoresists in the industry. However, cyclohexanone is still sensitive to light and non-neutral pH.
In addition, the solvents used must be environmentally acceptable. Cyclohexanone, discussed above, is considered to be barely tolerable by the industry due to its very low exposure limit. Given that 80% of all solvent used for spin-on processes, for example, is used at the clean up stage, including edge bead removal, wafer backside rinsing and spin-coater cup and nozzle rinsing, it is particularly important that the clean up solvent satisfy environmental considerations. It has also been recognized that solvents preferably should have sufficiently high flash points. For example the inorganic spin-on polymer of poly(perhydrido)silazane is conventionally formulated in dibutyl ether, a solvent with a flash point of only 25° C. Increasingly stringent environmental requirements place new constraints on solvents used with all of the dielectric materials, organic, organosilicon, and inorganic polymers alike.
As knowledge in the application and processing of dielectric materials expands, shortcomings among the currently-used solvents are becoming more recognized. It would be desirable to provide process compatible and benign solvents for a wide range of dielectric polymer materials used in the semiconductor industry. In particular, it would be desirable to provide a family of extremely useful high-boiling point solvents for formulation of dielectric polymer solutions, edge bead removal of these dielectric films, and process equipment rinsing.